The present invention relates to a method of forming an interlayer insulating film in a semiconductor device.
Known interlayer insulating films formed in semiconductor devices include a silicon oxide film, a silicon oxide film composed of an organic SOG (Spin-On-Glass) containing an organic component, and an organic polymer film.
In general, an interlayer insulating film formed in a semiconductor device is required to have a sufficiently low dielectric constant to achieve lower wiring capacitance and sufficiently high heat resistance to withstand a semiconductor manufacturing process.
With the increasing miniaturization of an LSI formed on a semiconductor substrate, wiring capacitance which is parasitic capacitance between metal wires has remarkably increased, while degraded performance of the LSI due to a wiring delay caused thereby has presented a serious problem. The wiring capacitance is determined by the size of a space between the metal wires and by the magnitude of the dielectric constant of an interlayer insulating film present in the space. To reduce the wiring capacitance, therefore, it is important to reduce the dielectric constant of the interlayer insulating film.
If an interlayer insulating film with low heat resistance is used, a thermal treatment at about 400.degree. C. performed in a semiconductor manufacturing process will soften the interlayer insulating film and undulate wiring, which may cause a fatal failure such as a disconnection or short circuit. This is why the interlayer insulating film is required to have sufficiently high heat resistance to withstand the thermal treatment at about 400.degree. C.
Since an interlayer insulating film composed of a silicon oxide film has an undesirably high dielectric constant, there has been proposed a fluorine-doped silicon oxide film made of silicon oxide doped with fluorine. Although the dielectric constant of the fluorine-doped silicon oxide film has been lowered by bonding a fluorine atom having low polarizability to a silicon atom composing the oxide film, the moisture absorbing property thereof increases with an increase in the amount of fluorine added thereto, so that a minimum dielectric constant attainable is about 3.5. Hence, it is difficult to use silicon oxide films including the fluorine-doped silicon oxide film as interlayer insulating films in an extremely miniaturized LSI.
In place of silicon oxide films, the use of an organic SOG film or organic polymer film as an interlayer insulating film in an extremely miniaturized LSI is under consideration because of its low dielectric constant.
The organic SOG film is formed by thermally curing a solution containing silica or siloxane each having an organic component such as a methyl group or a phenyl group. Since the organic component remains in the film even after thermal curing, a low dielectric constant of about 3.0 is attained.
As a first conventional embodiment, a method of forming an interlayer insulating film composed of an organic SOG film will be described with reference to FIGS. 6(a) to 6(d).
First, as shown in FIG. 6(a), first-level metal wires 2 are formed on a semiconductor substrate 1, followed by a first silicon oxide film 3 formed over the entire surface of the semiconductor substrate 1 including the first-level metal wires 2 by plasma CVD using a gas mixture of, e.g., tetraethoxysilane and oxygen as a raw material. Thereafter, an organic SOG agent is applied onto the first silicon oxide film 3 by spin coating and thermally cured to form an organic SOG film 4.
Then, as shown in FIG. 6(b), the entire surface of the organic SOG film 4 is etched back such that the portions thereof overlying the first-level metal wires 2 are removed.
Next, as shown in FIG. 6(c), a second silicon oxide film 5 is formed over the entire surface of the silicon oxide film 3 including the remaining organic SOG film 4 by, e.g., plasma CVD using a gas mixture of, e.g., tetraethoxysilane and oxygen as a raw material.
Next, as shown in FIG. 6(d), contact holes are formed in the first and second silicon oxide films 3 and 5 by using a resist pattern as a mask, which is then removed by using an oxygen plasma. Subsequently, a metal material is filled in the contact holes to form contacts. After that, second-level metal wires 7 are formed on the second silicon oxide film 5, resulting in a structure having an interlayer insulating film consisting of the first silicon oxide film 3, the organic SOG film 4, and the second silicon oxide film 5 between the first- and second-level metal wires 2 and 7.
As a second conventional embodiment, a method of forming an interlayer insulating film composed of a fluorinated amorphous carbon film, which is an organic polymer film, will be described. As disclosed in a technical report (Extended Abstracts of the 1995 International Conference on Solid State Devices and Materials, Osaka, 1995, pp.177-179), a fluorinated amorphous carbon film is formed by plasma CVD using, as a raw material, a mixture of a hydrocarbon-based component such as CH.sub.4 and a fluorine-containing component such as CF.sub.4.
After the gas mixture is introduced into a reaction chamber of a parallel-plate plasma CVD apparatus, the pressure inside the reaction chamber is held at several hundreds of Torr. When RF power on the order of 100 to 300 W at 13.56 MHz is applied to parallel-plate electrodes in the reaction chamber, the gas mixture is partially decomposed to generate monomers, ions, and radicals, which undergo plasma polymerization, resulting in a fluorinated amorphous carbon film as a plasma polymerization film deposited on a semiconductor substrate. The fluorinated amorphous carbon film thus formed has a low dielectric constant of 2.0 to 2.5 immediately after deposition.
However, since the foregoing organic SOG film is formed by repeatedly performing the steps of applying the organic SOG agent and thermally curing the applied organic SOG agent several times, it has the disadvantages of poor film formability resulting from a large amount of time required by the formation of the organic SOG film and high cost resulting from the major portion of the agent wasted during the spin coating.
In the case where the etch-back process, as illustrated in FIG. 6(b), is not performed with respect to the entire surface of the organic film 4 before contact holes are formed in the organic SOG film 4 and in the first silicon oxide film 3 by using the resist pattern as a mask, which is then removed by using an oxygen plasma, and contacts are formed by filling the metal material in the contact holes, the following problems arise. In the step of removing the resist pattern by using the oxygen plasma, SiCH.sub.3 contained in the organic SOG films 4 exposed at the sidewalls of the contact holes reacts with the oxygen plasma to generate SiOH, which is condensed by dehydration to generate H.sub.2 O in the step of filling the metal material in the contact holes. The resulting H.sub.2 O causes the oxidization and contamination of the metal forming the contacts, leading to faulty conduction at a contact.
As for the organic polymer film composed of the fluorinated amorphous carbon film, it has the advantage of an extremely low dielectric constant over the organic SOG film but is inferior thereto in heat resistance because of its low glass transition temperature. When the conventional fluorinated amorphous carbon film is subjected to a thermal treatment at a temperature of 300.degree. C. or more, the thickness of the film is significantly reduced, while the dielectric constant thereof is greatly increased. For example, if a fluorinated amorphous carbon film made from CH.sub.4 and CF.sub.4 and having a dielectric constant of 2.2 immediately after deposition is subjected to a thermal treatment at a temperature of 300.degree. C. for 1 hour, the film contracts till the thickness thereof is reduced to about 65% of the original thickness immediately after deposition, which is a 35% reduction, while the dielectric constant thereof is increased to about 2.8.
It is to be noted that the foregoing problems are not limited to the interlayer insulating film formed between upper and lower metallization layers but also arise in an interlayer insulating film between metal wires included in a single metallization layer.